Suggest pub(crate) imports

rust-analyzer has logic that discounts suggesting `use`s for private
imports, but that logic is unnecessarily strict - for instance given
this code:

```rust
mod foo {
    pub struct Foo;
}

pub(crate) use self::foo::*;

mod bar {
    fn main() {
        Foo$0;
    }
}
```

... RA will suggest to add `use crate::foo::Foo;`, which not only makes
the code overly verbose (especially in larger code bases), but also is
disjoint with what rustc itself suggests.

This commit adjusts the logic, so that `pub(crate)` imports are taken
into account when generating the suggestions; considering rustc's
behavior, I think this change doesn't warrant any extra configuration
flag.

Note that this is my first commit to RA, so I guess the approach taken
here might be suboptimal - certainly feels somewhat hacky, maybe there's
some better way of finding out the optimal import path 😅

crates/hir-expand/src/mod_path.rs

@@ -96,6 +96,10 @@ impl ModPath {
         self.kind == PathKind::Super(0) && self.segments.is_empty()
     }
 
+    pub fn is_pub_crate(&self) -> bool {
+        self.kind == PathKind::Crate && self.segments.is_empty()
+    }
+
     #[allow(non_snake_case)]
     pub fn is_Self(&self) -> bool {
         self.kind == PathKind::Plain